Effect of Defect on the Photoelectrochemical Performance of Amorphous ALD-TiO x Thin Films

Kim, Min Ji 부산대학교 대학원 2024 국내석사

The unchecked use of fossil fuels is accelerating global warming due to the greenhouse effect. Finding new technologies to harvest energy from renewable sources is crucial for a sustainable civilization. One promising alternative is generating hydrogen fuel from solar energy and water via the photoelectrochemical (PEC) water splitting process. Titanium dioxide (TiO2) has been a promising material for PEC applications for several decades, especially defective TiO2 has shown higher PEC performance due to its excellent chemical and electrical properties. In this study, we used atomic layer deposition (ALD) with TDMAT as a precursor for Ti to deposit amorphous titanium oxide (TiOx) films on both Si wafers and FTO-coated glass substrates. We performed focused ion beam (FIB) studies with Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) cross-sections to verify the thickness and defect concentration of small ALD-TiOx film layers within the TiOx film matrix. Additionally, we evaluated the film's optical absorption properties by conducting UV-Vis absorption spectroscopy. The defect concentration of the TiOx film was adjusted by changing the chamber temperature during the ALD process. The TiOx film deposited at 200 °C showed a higher defect concentration than other films, resulting in a higher photo absorption efficiency (~50.41 %), photo-generated charge separation efficiency (~33.31 % at 1.23 V vs. RHE), and charge transfer efficiency (~80.33 % at 1.23 V vs. RHE), and a photocurrent density of 0.051 mA/cm2 at 1.23 V vs. RHE. Finally, we systematically correlated the defect concentration of the amorphous TiOx film with its PEC activity. This study provides insights into the detailed PEC characteristics of the defective amorphous ALD-TiOx films.

The dielectric and optical properties of thin films at gigahertz and terahertz frequencies

이광수 Rensselaer Polytechnic Institute 2002 해외박사

Thin films have attracted the attention of physicists because the dimensionally constraint system of the thin films can create an intermediate system between macroscopic systems and molecular systems. This system might show anomalies in the optical, electrical, mechanical, or magnetic properties, compared to the properties of bulk materials. The most conspicuous phenomena associated with the thin films are the anomalies of dielectric and optical properties. This thesis will investigate the anomalous phenomena of thin films by the study of the dielectric and optical properties of a variety of thin films at GHz and THz frequencies (in the millimeter and sub-millimeter wave regime). A coherent THz wave source and electro-optic detection method are used for the experiment. For a coherent THz frequency source, a THz emitter was used, which was driven by a 100 femto-second pulse from a Ti:sapphire laser to produce the coherent THz wave radiation while the electro-optic detector was gated by the laser pulse split from the laser for the coherent THz wave detection. The dielectric and optical properties of a variety of thin film materials are discussed and analyzed in detail. The microscopic structural contribution to the dielectric and optical properties of thin films are especially emphasized in this thesis. Because the currently available methods for the characterization of thin films at the THz frequency have difficulties when the films are in the range of nanometer to micron in thickness, a new method, THz differential time-domain spectroscopy, is developed. The theory of the THz differential time-domain spectroscopy is also developed to obtain information on the real and imaginary parts of dielectric properties and the optical properties. The dual phase lock-in detection was performed to measure the very small differential THz signal with a high dynamic range up to 105. This THz differential time-domain spectroscopy is able to characterize the properties of nanometerscaled thin films. A variety of materials have been experimentally investigated, including a 930㎚ silicon dioxide film, a 1.8 ㎛ parylene-n polymer free standing film, a 1.8㎛ parylene-n polymer film on silicon substrate, a 300㎚ parylene-n polymer film, a 100㎚ tantalum oxide high dielectric film, protein monolayers, water molecule layer on the surface of ice crystal, and a carbon nanotube film. We found that there are anomalies in the dielectric and optical properties of several thin films. These anomalous behaviors are believed to be caused by fine grain formation, mechanical stresses, formation of interfacial layers, or rough interfaces during thin film deposition processes.

Growth of MgB2 thin films and study on their vortex dynamics

Pham, Duong Sungkyunkwan university 2019 국내박사

The conventional superconductor MgB2, which was discovered in 2001, shows some advantages compared to other superconductors, including relatively high critical temperature (Tc) of 40 K, high self-field critical current density (Jc), simple crystal structure, low cost, and so on. However, its rapid drop of Jc in magnetic field limits its practical applications. Numerous researches on both fundamental physical properties and practical applications have been conducted on this material. Among them, the study on vortex dynamics have attracted considerable interest, since the vortex’s movement is main reason for the current dissipation. In this work, we have focused on the study of vortex dynamics with respect to the pinning forces, thereby, comprehend how to improve Jc of MgB2 thin films. Moreover, the transverse movement of vortex was investigated through the Hall effect. Firstly, the fabrication of high-quality MgB2¬ thin films, however, also a big challenge. The hybrid physical-chemical vapor deposition (HPCVD) was known as a very effective method for the deposition of MgB2 thin films. By optimizing the growth condition, we have fabricated high-quality MgB2 films at both low (~ 480o C) and high temperatures (~ 700o C). All films were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), physical property measurement system (PPMS) and magnetic property measurement system (MPMS). The films fabricated at low temperatures were used to investigate the effect of ZnO nanoparticles impurity on flux pinning strength, since low growth temperature can avoid the decomposition of nanoparticles and the formation of undesirable phases. MgB2 thin films grown on ZnO seeds impurity layers showed a significant enhancement of Jc in the magnetic field due to the creation of additional pinning sources, namely point defects and grain boundaries. The broad peaks were observed in the magnetic field dependence of the flux pinning force density, indicating competition of different pinning sources. In the focus part of this work, we studied the effects of ion irradiation on the superconducting properties of single-crystalline MgB2 thin films fabricated at high temperatures. The low irradiation energy was used to implant oxygen (O) ions into the films with various thicknesses (130, 410, 850, and 1,300 nm). The correlation thickness of the top-implanted layer and thickness of the bottom-clean layer could be adjusted by controlling irradiating energy. An interesting exponential dependence of Tc on the thickness ratio of these layers was found possibly due to conventional proximity effect. The MgO, which is known as an effective pinning source for MgB2, was induced in implanted films. Thus, despite the use of low irradiation energy, the field performance critical current density, Jc(H), was significantly improved in most cases of irradiated films. The pinning force density (Fp) showed a crossover from surface pinning to normal point pinning mechanism. Furthermore, we have studied the angle dependent Hall effect in single-crystalline MgB2 thin films prior to and after Fe ion irradiation. The angle dependent Hall scaling ρxy = A ρβxx shows a universal behavior, as scaling exponent β = 2 ± 0.2 holds even after various doses of irradiation. By applying the anisotropic mass scaling model, the magnetic field dependent resistivity curves measured under various angles were shown to potentially overlap on a main curve, except for the small angle region (called failed angles). We found that the failed angles were decreased with increasing irradiation doses. This behavior could be well interpreted within the frame of theories on the Hall effect and vortex dynamics.

From Thin Films to Nanoparticles : Investigation of Polymerization Processes in Capacitively Coupled Hydrocarbon Plasmas

홍석호 Ruhr-Universitat Bochum 2004 해외박사

In recent years, low temperature plasmas have received great attention mainly in the different fields of plasma processing. Different types of plasmas such as glow-, arc-, RF-, and microwave plasmas have been used for deposition of thin films, for etching, and for various types of surface treatments. In order to develop new industrial applications and to improve existing products, investigations of plasma-wall interactions and plasma polymerization are necessary. Based on the knowhow and knowledge obtained by researches, more precise and controlled deposition/etching techniques can be achieved. One important problem for the application of reactive low temperature plasmas is the formation of nanoparticles in the plasma volume. It is well known that such particles may act as "killer particles" which are responsible, for example, for the malfunction of integrated circuits. On the other hand a controlled growth of particles is desired for applications in catalysis and pharmacy and for the fabrication of nano-crystalline materials. In the latter case nanoparticles are incorporated into the growing film for a controlled modification of film properties. Especially in gas mixtures which are used for thin film deposition, polymerization processes in the plasma volume - leading to the formation of particles - are competing with "surface polymerization" processes which are leading to the formation of thin films. In this work both processes were studied in a capacitively coupled GEC Cell in mixtures of argon and different hydrocarbon gases. Capacitively coupled discharges are commonly used for deposition and etching of thin films. On the other hand they are also an appropriate tool for the study of polymerization processes in the plasma volume since they guarantee a very good con?nement of negatively charged clusters which is a precondition for the growth of nano- or micro-sized particles. An important parameter for such kind of discharges is - beside the choice of the precursor gas - the rf power. Depending on power input and power coupling two types of hydrogenated carbon films with di�erent mechanical and optical properties can be deposited: polymer-like and diamond-like carbon ?lms. The polymer-like films have low refractive index, low carbon density and a low hardness. The diamond-like films have, on the other hand, high refractive index, high carbon density and a high hardness. The decisive parameter which determines the properties of the deposited films in these experiments is the energy of the ions that are impinging on the substrate. The importance of the ion energy for the properties of amorphous hydrogenated carbon ?lms (i.e. refractive index, hardness, carbon density, etc) was also nstrated in another discharge type. Experiments performed in an expanding microwave plasma confirm that refractive index and carbon density can be drastically increased by applying an external (pulsed DC) bias, i.e. by increasing the ion energy. Another important factor is the choice of the precursor gas. The experiments show that the film growth is much faster in C2H2 plasmas than in CH4 plasmas. A similar result is obtained for the formation of nanoparticles in the plasma volume. The particle growth is much faster in C2H2 plasma than in CH4 plasmas. oreover the initiation of the particle growth is di�erent for both kind of gases. For identical process conditions the particles are formed spontaneously in the Ar/C2H2 discharge whereas in the Ar/CH4 discharge the formation of particles could be observed only either after a certain amount of C2H2 (from an external source) was transiently added to the discharge at a constant low input power or transient high input power at the ignition phase. From these experiments it can be concluded that the presence of a critical density of C2H2 is a decisive parameter for the initiation of the particle growth. This is a very important result, since the initial phase of the particle formation can be detected and controlled by monitoring the C2H2 amount in hydrocarbon containing plasmas. The formation of particles was studied by two di�erent methods: one direct method and one indirect method. The direct method is based on laser light scattering and gives information about the temporal and spatial evolution of the dust formation. The indirect method is based on the fact that the plasma parameters (electron density , electron temperature, light emission from the plasma, etc) are influenced by the presence of dust particles. Although this response of the plasma to the formation of particles gives only global information about the presence of particles it can be used as a tool for process controlling. It is known for example that the presence of dust particles in the discharge has a dramatic e�ect on the discharge impedance [92]. Consequently the Fourier pectrum of the voltage that is applied to the upper electrode is strongly influenced by the formation of particles. Especially the 7th harmonic of the voltage signal was found to correlate strongly with the presence of particles in the discharge chamber which was cross checked with Mie-ellipsometric measurements. The results show that the measurement of the 7th harmonic can be used as a sensitive method for the detection of dust particles. With respect to applications in the microelectronic industry the detection of very small killer particles is an important precondition for the development of future devices. The use of the 7th harmonic is a possible alternative for methods based on laser light scattering which cannot be applied for very small particles, since the scattered laser light intensity is proportional to r6. For given process parameters (pressure, gas mixture, etc) this method can be used even as an indicator for the temporal development of the particle size. An application which combines the formation of particles in the plasma volume and the deposition of thin films on a substrate is the controlled incorporation of carbon-nanoparticles in DLC ?lms. Despite the useful properties of DLC ?lms such as wear resistance, smoothness, low friction and bio-compatibility they have one important disadvantage. Their internal stress can reach up to several GPa when the film is thicker than a few hundred nm. As a result the stability of the film is dramatically reduced. One possibility to overcome this problem is to embed hydrogenated carbon particles into the ?lm. First experiments show that the internal stress of DLC ?lms was indeed reduced due to the incorporation of nano particles. Further experiments especially for the quanti?cation of this e�ect have to be done in the future. Another interesting and still open problem for bio-medical applications is the control of nanoporosity of DLC ?lms. The nanoporosity can be measured by using a positron scanning microscope, i.e. characterization of the defects via positron annihilation life time [137]. Our preliminary results [138] show that the positron life time in polymer-like ?lms is relatively long, about 1700-1900 ps. It is found that these ?lms have similar structure as polyethylene [139,140]. The positron life time in diamond-like ?lms is short, about 300-400 ps, which is a typical positron life time in graphite with large defects [137,141]. These results are consistent with previous research of others [137,139,140,142] on carbon materials. Another interesting application would be incorporation of foreign nanoparticles into DLC films to change their electrical properties. DLC films can have either p- or n-type characteristics depending on hydrogen contents in the film [143]. Combining with metal or emiconductor-like nanoparticles, one might be able to enhance the field emission properties, for example.

Bi0.5Na0.5TiO3-BaTiO3계 압전박막의 합성, 특성평가 및 알칼리 금속의 도핑효과

아카리아쿠마수 전북대학교 일반대학원 2013 국내박사

Due to the toxicity of lead, there is an urgent need to develop lead-free alternatives to replace the currently dominant lead-based piezoelectrics such as lead zirconate titanate (PZT). Among all the lead–free piezoelectric materials the solid solution of Bi0.5Na0.5TiO3 and BaTiO3 (BNT–BT) shows excellent piezoelectric and ferroelectric property owing to the morphotropic phase boundary (MPB) formed between its respective end members: the rhombohedral Bi0.5Na0.5TiO3 and the tetragonal BaTiO3 perovskites. This materials system have been extensively studied in bulk ceramic form, however many of the ultimate applications will be in thin film embodiments (i.e., microelectromechanical systems). For this reason, in this thesis these lead‐free piezoelectrics are synthesized in thin film form to understand the structure‐property‐processing relationships. Fabrication of high quality thin films on Si/SiO2/Ti/Pt substrate was attempted by metal organic solution deposition method. Annealing and processing conditions were varied to optimize the microstructure and piezoelectric properties of thin films. To improve the piezoelectric performance of BNT–BT thin film, we performed alkali metal doping on BNT–BT solid solutions. It was found that Li doping reduced the coercive field and improved the piezoelectric properties of the BNT–BT thin films and optimized performance was found at 10% Li. The reasons for the improved piezoelectric properties and decrease in coercive field were discussed. As compared to binary BNT–BT, the Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3 (BNT–BKT–BT) ternary system has shown enhanced piezoelectric properties at MPB in bulk form. Literature shows that this zone is not well studied in thin film form and more works are needed to determine more precisely this boundary. In this work, several compositions in the BNT–BKT–BT system have been studied in order to obtain the optimum piezoelectric and ferroelectric properties. Out of all thin films, the composition 0.884BNT–0.036BT–0.08BKT has the most optimized electrical properties. Following a series of studies on effect of Rb doping on BNT–BT thin films, it was revealed that addition of 5 mol% Rb to BNT–BT composition resulted in a significant suppression of leakage current and major enhancement of dielectric, ferroelectric and piezoelectric properties. The results of this study present the great potential of and BNT–BT thin films for piezoelectric MEMS devices and provide a baseline for future investigations on lead–free piezoelectric thin films. 납의 독성 때문에, 특성이 양호한 lead zirconate titanate(PZT)와 같은 현재 납을 기반으로 하는 압전재료를 대체하기 위해 비연계 압전재료를 개발하기 위한 필요성이 대두되어 왔다. 지금까지 연구된 비연계 압전재료 중에서 Bi0.5Na0.5TiO3와 BaTiO3(BNT–BT)의 고용체는 상(능면체 Bi0.5Na0.5TiO3과 정방정 BaTiO3)의 경계(MPB)에서 우수한 압전특성과 강유전특성을 나타낸다. 이러한 비연계 압전재료는 폭넓게 벌크 세라믹 분야에서 연구되었지만, 궁극적인 적용은 박막을 이용한 통합 시스템(즉, 마이크로 전기자동 시스템) 등에서 있을 것이다. 이러한 이유로, 이 졸업논문에서 박막으로 형성된 비연계 압전재료는 구조–성능–공정 관계를 이해하기 위해서 합성되었다. 높은 품질의 박막 제작은 Si/SiO2/Ti/Pt 기판 위에 금속유기물 용해 증착법에 의하여 이루어졌다. 어닐링과 공정조건은 박막의 미세조직과 압전특성을 최적화하기 위해 조정되었다. BNT–BT 박막의 압전특성을 개선하기 위해, BNT–BT 고용체에 알칼리금속을 도핑시켰다. Li 도핑은 잔류분극을 향상시키고, 항전계를 감소시켜, BNT–BT 박막의 압전특성을 향상시켰으며 최적화된 성능은 10mol% Li에서 찾아볼 수 있었다. Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3(BNT–BKT–BT) 삼원계는 이원계인 BNT-BT보다 MPB 영역에서 향상된 압전특성을 나타는데, 0.884BNT–0.036BT–0.08BKT 조성이 최적화된 전기적 특성을 가진다. 또한, BNT-BT 박막에 Rb 도핑효과의 일련의 연구에 따르면, BNT–BT에 5mol% Rb의 첨가는 유전특성, 강유전특성, 압전특성 증가의 결과를 보여준다. 이러한 연구결과는 큰 잠재력을 가지는 BNT–BT 박막에 대한 압전 MEMS 디바이스와 비연계 압전박막의 산업화를 위한 기초적인 베이스라인을 제공한다.

Preparation of High-Functional Thin Films on Particles by PCVD Process and Its Application to Pollutant Removal

팜흥크엉 강원대학교 대학원 2012 국내석사

There are several types of reactor for dry particle coating, such as chemical vapor deposition reactor, molecular beam epitaxy reactor, spray pyrolysis reactor, fluidized bed reactor. The plasma chemical vapor deposition (PCVD) process has been widely used for preparation of high quality thin films and can also be used to coat the uniform thin films on particles. In the rotating cylindrical PCVD reactor, the particles will rotate with the cylindrical PCVD reactor. The active radicals for particle coating are generated by the plasma. Some particles will be coated with the active chemicals while they stay on the cylindrical tube wall. The other particles will stay in the gas phase of bulk plasma to coat uniform films on the particles. In this study, we coated TiO2 thin films, SiOx thin films, TiO2/SiOx double-layer films on PP particles and glass beads using the rotating PCVD reactor. We investigated the effects of process variables on growth of thin films onto the particles. The photocatalytic activity of TiO2 thin films on PP beads was tested on the photodegradation of phenol in aqueous solution. The photodegradation rate of phenol by TiO2 thin films on the PP beads increases, as the initial phenol concentration increases or as the number of PP beads coated with TiO2 thin films increases in aqueous solution. The NO and SO2 removal efficiencies can be enhanced by using a combination of dielectric barrier discharge and photodegradation by TiO2 single-layer films or TiO2/SiOx double-layer films. The stronger the applied voltage is, the higher the pulse frequency is, or the longer the gas residence time is, the higher the NO and SO2 removal efficiencies become. We also investigated the effect of thickness of TiO2 thin films and the effect of SiOx thickness of TiO2/SiOx double-layer films onto glass beads for NO and SO2 removal by dielectric barrier discharge-photocatalyst (DBD-P) hybrid process.

Defects engineering of perovskite thin films for flexible perovskite solar cells

Gong, Ohyeong Sungkyunkwan University 2023 국내박사

현재, 세계적으로 온실 가스 배출을 줄이고 재생 가능한 에너지원의 사용을 통한 기후 변화를 해결하는 것에 중점을 두고 있다. 풍부한 태양 에너지를 전기로 변환하는 태양광 발전은 많은 관심을 주목받으며 발전 비중이 더욱 높아질 것으로 전망된다. 태양전지는 재료와 공정 기술의 지속적인 발전으로 성능이 향상되고 무게가 감소되었다. 더불어, 태양전지의 다양한 응용과 활용을 위해 수요가 더욱 높아지고 있으며, 고성능, 경량화, 유연성을 제공하는 차세대 박막 태양전지의 개발은 재생 가능한 태양 에너지를 활용하는 실용성을 더욱 향상시킬 것으로 기대되고 있다. 페로브스카이트 태양전지는 페로브스카이트 구조의 물질을 광흡수층으로 사용하는 태양전지를 통칭한다. 페로브스카이트 광흡수체의 우수한 광학적, 전기적 특성을 통해 신흥 태양전지 소재 중 가장 높은 광전 변환 효율을 보이며, 광범위한 연구들이 진행되고 있다. 또한, 페로브스카이트 태양전지는 비교적 비용이 저렴한 재료와 용액 공정을 통해 생산될 수 있어 생산성이 높으며, 박막 구조의 물리적 특성에 의해 우수한 기계적 내구성과 경량성을 나타낼 수 있다. 그러나 용액 공정을 통해 빠르게 형성되는 페로브스카이트 박막은 0차원 점결함부터 3차원 체적결함까지 다양한 결함이 형성되기 쉽다. 이러한 결함은 전하 트랩으로 작용하고 페로브스카이트를 구성하는 이온의 이동을 촉진시켜, 페로브스카이트 태양전지의 성능 저하를 초래하고 낮은 안정성을 나타낸다. 더불어, 현재까지 수행된 대부분의 연구는 유리 기판 기반으로 수행되어 페로브스카이트 박막의 장점을 살리지 못하고 있다. 본 논문에서는 효율적이고 안정적인 유연 기판 기반 페로브스카이트 태양전지의 개발에 기여하기 위해 페로브스카이트 박막의 제작에서 재생산에 이르기까지 발생할 수 있는 결함들을 제어하기 위한 새로운 접근법들을 소개하였다. 제2장에서는 유연 기판 상에 고품질의 페로브스카이트 박막을 형성하기 위해 판데르발스힘 보조 열전달 공학을 소개하였다. 개발된 공정을 통해 유연 기판 상에 유리 기판 상과 유사한 품질의 고품질 페로브스카이트 박막을 형성할 수 있었으며, 대면적 페로브스카이트 박막 공정에서도 효과가 우수한 것을 입증하였다. 또한, 유연 기판 기반의 태양전지가 기존의 유리 기판 기반의 태양전지보다 낮은 성능을 가지는 원인은 기판의 고유 특성에만 의존된다는 것이 조사되었다. 제3장에서는 페로브스카이트 박막의 표면 결함을 효과적으로 억제하기 위해 진공 보조 자기 조립 패시베이션 공학을 제시하였다. 개발된 공정 기술을 통해 패시베이션 물질을 결정립계 말단까지 침투시켜 페로브스카이트 결정의 완전한 패시베이션을 유도하였고, 전기적 특성을 저해할 수 있는 잔여 패시베이션 물질을 효과적으로 제거하여 고성능 유연 페로브스카이트 태양전지를 제작하였다. 또한, 초경량 유연 태양광 미니 모듈 제작을 통해 우수한 유연성 및 경량성을 선보일 수 있었다. 제4장에서는 페로브스카이트/페로브스카이트 박막 간의 적층 계면에서의 결함을 억제하기 위해 새로운 양면 수광형 유연 페로브스카이트 태양전지 제조 공정을 소개하였다. 페로브스카이트 박막의 표면 거칠기를 극성도가 적절한 용매의 후처리 (화학적 연마)를 통해 완화시킬 수 있었고, 이로 인해 페로브스카이트/페로브스카이트 적층 계면에서의 형성될 수 있는 결함을 효과적으로 억제할 수 있었다. 결함 제어를 통해 세계 최고 수준의 양면 수광형 유연 페로브스카이트 태양전지를 선보일 수 있었다. 제5장에서는 고품질 페로브스카이트 박막을 재생산하기 위해 페로브스카이트 태양전지의 재활용 공정에서 사용된 용매를 재사용할 수 있는 새로운 공정 방법을 제안하였다. 페로브스카이트 태양전지의 재활용 공정에서 사용된 용매 내 불순물의 종류와 농도 및 각 잔여 불순물들이 페로브스카이트 박막에 미칠 수 있는 영향에 대해 조사하었다. 조사 결과를 바탕으로, 페로브스카이트 태양전지의 재활용 공정에서 선택적 용해를 통해 각 구성 물질을 효과적으로 회수하고 재사용하여 고품질 페로브스카이트 태양전지를 재생산할 수 있었다. 이러한 혁신적인 접근 방식들이 효율적이고 안정적인 유연 페로브스카이트 태양전지의 발전에 기여하여 보다 지속 가능하고 재생 가능한 에너지를 사용하는 미래에 가까워질 수 있기를 기대한다. Currently, there is a global emphasis on reducing greenhouse gas emissions and addressing climate change through the use of renewable energy sources. Photovoltaic cells, also known as solar cells, which convert abundant solar energy into electricity, are gaining significant attention in this regard. Solar cells have continuously advanced in terms of materials and process technologies, resulting in improved performance and reduced weight. They are now being utilized not only in satellites but also in everyday applications such as buildings and vehicles. The development of next-generation thin film solar cells, offering higher efficiency, lighter weight, and flexibility, is expected to further enhance the practicality of utilizing renewable solar energy. Perovskite solar cells (PSCs) have garnered extensive research focus among emerging solar cell technologies due to their demonstrated highest efficiency. PSCs utilize perovskite-structured materials as the light-absorbing layer, enabling high efficiency through their excellent optical and electrical properties. Moreover, PSCs offer the advantage of cost-effective materials and solution processes, resulting in increased productivity. Additionally, PSCs exhibit exceptional mechanical durability and are remarkably lightweight thanks to the physical properties of their thin films. However, perovskite thin films, rapidly formed through solution processes, are prone to various defects ranging from zero-dimensional point defects to three-dimensional bulk defects. These defects act as charge traps and impact the movement of ions, leading to performance degradation. Furthermore, the majority of studies conducted thus far have employed rigid glass-based substrates, limiting the full utilization of the advantages offered by perovskite thin films. In this thesis, novel approaches are introduced to address the issue of defects in perovskite thin films, from their production to recycling, with the aim of facilitating the development of efficient and stable flexible PSCs (F-PSCs). In Chapter 2, the quality of perovskite thin films on flexible substrates can be enhanced by implementing a novel heat-transfer engineering approach during the formation of the thin films. This approach focuses on controlling perovskite grain growth and improving the overall quality of the perovskite thin films. In Chapter 3, surface defects in perovskite thin films can be effectively suppressed through a novel post-treatment engineering method. This method aims to mitigate surface defects and improve the overall quality of the perovskite thin films, resulting in enhanced performance and stability of the F-PSCs, including large-area F-PSCs. In Chapter 4, a novel process for fabricating bifacial F-PSCs is introduced, which inhibits defects at the lamination interface between perovskite/perovskite films. This approach enhances the efficiency and reliability of the bifacial F-PSCs by minimizing defects during the lamination process. In Chapter 5, a systematically designed recycling process is suggested to re-produce high-quality perovskite thin films, enabling a closed-loop cycle for PSCs. This recycling process aims to effectively recover and reuse materials, contributing to the sustainability and environmental friendliness of the technology. These innovative approaches are expected to contribute to the advancement of efficient and stable F-PSCs, bringing us closer to a more sustainable and renewable energy future.

광전소자 응용을 위한 (Cd,Zn)ZnO 박막의 밴드갭 엔지니어링 및 광발광에 관한 연구 : Bandgap engineering and photoluminescence properties of (Cd,Mg)ZnO film for optoelectronic devices

지익수 인제대학교 일반대학원 2016 국내석사

ZnO, a wide band gap (3.37 eV) and large exciton binding energy (60 meV) material, has drawn much attention for applications in optoelectronic devices. This dissertation describes an experimental study on synthesis and characteristics of (Cd,Mg)ZnO film. Fitstly, Ternary CdxZn1-xO thin films are deposited using a sol-gel dip-coating method onto quartz substrates with various amounts of Cd content (x = 0, 0.05, 0.15, and 0.25). The structural and optical properties of the CdxZn1-xO thin films are investigated using x-ray diffraction (XRD), photoluminescence (PL) and ultraviolet-visible spectroscopy. In the XRD patterns, the intensity of the diffraction peaks for ZnO decreases, while the intensity of the diffraction peaks for CdO increases with increases in the Cd content. Above x = 0.15, both diffraction peaks for ZnO and CdO are observed in the XRD pattern, and this indicates that the wurtzite structure for ZnO and the rock-salt structure for CdO coexist in CdZnO thin films. The PL spectra of the films results demonstrate that the near band edge emission peaks in the ultraviolet region shift to lower energy range (red-shift) and the deep level emission peaks at the visible region decrease with increases in the Cd content. The optical properties such as transmittance, optical band gap, and Urbach energy are calculated using the optical data. Using linear fitting of the absorption edge, the band gap energies of the thin films are derived as 3.27, 3.19, 3.12, and 3.03 eV for x = 0, 0.05, 0.15 and 0.25, respectively. In addition, the bowing parameter for the energy band gap of CdxZn1-xO is estimated to be Eg(x)=3.3-1.2x+x^2 . Secondly, we fabricate magnesium zinc oxide (MZO) thin films inserting a buffer layer between a film and a substrate with different annealing temperatures. The structural and optical properties of MZO thin films are investigated by using scanning electron microscopy, X-ray diffraction (XRD), and photoluminescence. The XRD patterns for the buffer layers annealed at 200 oC show the narrowest full width at half maximum and the most intense (002) diffraction peak. In addition, the buffer layers decrease the magnitude of the residual stress between the MZO thin film and the silicon substrate. The near-band-edge emission peaks intensify and the deep-level emission peaks significantly intensify with increasing buffer layer annealing temperatures. Inserting a buffer layer between a film and a substrate improves the crystallinity of MZO thin films. Finally, the vapor phase transport (VPT) process is usually used for the growth of one-dimensional nanostructures rather than for film deposition. In this study, for the first time, we report on the fabrication and optical propeties of ZnO film produced by VPT combined with rapid heating. The X-ray diffraction results show that, the intensity of the ZnO (002) peak was largest, indicating that the c-axis orientation of the ZnO grains was perpendicular to the substrate. In the photoluminescence (PL) spectra of the ZnO film at 12 K, PL peaks were observed at 3.374, 3.362, 3.319, and 3.242 eV, which were attributed to free excitons (FXs), excitons bound to neutral donors, donor-acceptor pairs (DAPs), and first-order longitudinal optical phonon replicas of DAPs (DAPs-1LO), respectively. The energy interval between DAPs and DAPs-1LO was almost the same as the longitudinal optical (LO) phonon energy of bulk ZnO of ~76 meV. The PL spectra in the temperature range from 12 to 300 K shifted to lower energy with increasing temperature. The activation energy of FX was estimated to be 61 meV. This value is similar to the exciton binding energy of 60 meV.

음극아크법으로 증착된 금속질화물 박막에 대한 포유류 세포의 시험관내 반응 : In Vitro Mammalian Cell Response to Cathodic Arc Deposited Metal Nitride Thin Films

Pham Hung Vuong 울산대학교 2009 국내박사

Osteoblasts cell response to TiN, TiAlSiN, TiCrAlSiN, CrN, CrAlSiN and ZrAlSiN thin films and Ti thin films was evaluated in vitro. In this work, cell adhesion, actin cytoskeleton, microtubule organization and focal contact adhesion as well as cell proliferation were investigated. Cell adhesion, actin stress fibers and microtubule organization were significant difference on the (TiAlSiN, TiCrAlSiN, CrN, CrAlSiN thin films), and the Ti thin film whereas cell ashesion, actin stress fiber and microtubule organization were slightly difference on TiN thin film and Ti thin films. Immunofluorescent staining of vinculin in osteoblasts cell also showed various focal no focal adhesions were detected on the CrN thin films. The TiAlSiN, TiCrAlSiN, CrAlSiN and ZrAlSiN thin films showed significant greater cell proliferation in relation to the glass surface or Ti thin films whereas the TiN thin film displayed a slightly higher cell proliferation compared to that on glass surface or Ti thin film. Inconstrast, the CrN thin films showed significant lower cell adhesion and proliferation in relation to glass surface or Ti thin films. As a result, the TiN, TiAlSiN, TiCrAlSiN, CrAlSiN and ZrAlSiN thin film could be a potential candidate as a tribological coating to be used for supporting hard tissue on implant surfaces whereas the CrN thin film could be a promising hard thin film to be used for inhibiting tissue growth on implant surfaces.

Functional multilayer films with controlled nanostructures for in vitro cell studies

장영선 서울대학교 대학원 2013 국내박사

Functional polymer thin films are of great interest for diverse potential applications due to their excellent productivity and cost-efficiency. Thorough understandings on the relationship between structures and physicochemical properties of polymer thin films play pivotal roles in developing the films for the fine purpose. Among many different types of functional polymer thin films, the multilayer films with controlled nanostructures has been recently highlighted in many biomedical applications such as controlled release platforms, disease diagnosis platforms, and tissue engineering. Particularly, in order to develop such novel biomedical platforms based on the functional multilayer films, in vitro cell study has to be performed, which provides perspectives on regulating the cell behaviors. In this thesis, we focus on the layer-by-layer (LbL) strategy for fine tuning of structures and properties of polymer thin films as well as for constructing polymer platforms for in vitro studies on metastatic cancer cell behavior. In particular, the external stimuli-triggered release mechanism of polymer multilayer films has been systematically investigated, in terms of changes in internal structures and physicochemical properties, in order to establish the ground for designing the controlled release of target active materials from thin film coatings. Furthermore, the functionalization of multilayer films with controlled nanostructures has been carefully examined in the viewpoint of the control of cell-matrix or cell-cell interactions. In Chapter 1, the control strategies for engineering internal structures and swelling properties of polymer multilayer films are introduced on the basis of the LbL deposition, because it has been the one of the most efficient methods for preparing functional multilayer platforms, taking advantage of various intermolecular interactions among paired species. The growth rates of bilayer thickness and the internal structures in multilayer films were greatly controlled by tuning the range of the intermolecular interactions between polymer chains (i.e., long-range electrostatic interactions or short-range hydrogen bonding) and by the film fabrication methods (i.e., dip- or spin-assisted LbL deposition). The internal structures of multilayer thin films in nanometer scale were systematically investigated by the neutron reflectivity (NR) measurements, as a function of LbL deposition techniques and the types of intermolecular interactions. Furthermore, it was demonstrated that the loop and tail conformations of partially charged weak PE chains preferentially capture water molecules within multilayer films as compared to fully charged and tightly bound PE chains with stretched conformations by NR along with quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. In Chapter 2, we have designed the controlled release platforms based on polyelectrolyte (PE) blend multilayer films to investigate the release mode and kinetics at the nanoscale level. The model blend multilayer films are composed of positively charged layers with weak PE (linear poly(ethylenimine), LPEI) and negatively charged blend layers with mixtures of strong (poly(sodium 4-styrenesulfonic acid), PSS) and weak (poly-(methacrylic acid), PMAA) PEs. The blend multilayer films ([LPEI/PSS:PMAA]n) with well-defined internal structure were prepared by the spin-assisted LbL deposition method. The changes in nanostructures and physicochemical properties of the blend multilayer films were systematically studied as a function of blend ratio by NR, ellipsometer, AFM, FT-IR spectroscopy, and QCM-D. Since PSS strong PEs serve as robust skeletons within the multilayer films independent of external pH variation, the burst disruption of pure weak PE multilayer films was dramatically suppressed, and the release kinetics could be accurately controlled by simply changing the PSS content within the blend films. These release properties of blend multilayer films form the basis for designing the controlled release of target active materials from surfaces. In Chapter 3, we present the effect of molecular weight (MW) of PEs on the disintegration behavior of weak PE multilayer films consisting of LPEI and PMAA. The multilayer films prepared by the spin-assisted LbL deposition have well-ordered internal structures and also show the linear thickness growth behavior regardless of MWs of PMAA. The well-defined weak PE multilayer films were subject to disintegration into bulk solution when the electrostatic interactions between LPEI and PMAA layers were reduced by treatment at pH 2. However, we demonstrated the change in the disintegration mode and kinetics (i.e., from burst erosion to controlled surface erosion) as a function of MW of PMAA based on neutron reflectivity (NR) and quartz crystal microbalance with dissipation (QCM-D), revealing the correlation between the structural changes and the viscoelastic responses of the weak PE films upon pH treatment. Also, the unique swelling behavior as well as the significant increase in dissipation energy was monitored before the complete disintegration of the multilayer films containing high MW PMAA, which is believed to originate from their slow rearrangement kinetics within the film. We believe that the results shown in this study provide chain-level understanding as to the MW-dependence on pH-triggered disintegration mechanism of weak PE multilayer films. In Chapter 4, we have developed in vitro platforms for studying metastatic cancer cell behavior, based on the surface modification of LbL-assembled multilayer films with functional nanoparticles and biomolecules. The polymer multilayer platforms prepared by spin-assisted LbL deposition provide controlled surface charge and stable mechanical property in cell culture environments, which could offer easy surface modification with charged functional molecules while creating intimate cell-surface contacts. Gold nanoparticles (AuNPs) were employed to modify flat LbL surfaces and investigated the effect of nano-topographical cues on the metastatic cancer cell focal adhesion, shape and motility. Moreover, cellular signaling process with proteins in extracellular matrices (ECMs) was mimicked and analyzed by incorporating biomolecule-conjugated AuNPs onto LbL films. As a result, it is confirmed that the existence of nanotopographical features with a cell adhesion protein (fibroectin, Fn) is critical in inducing dramatic changes in metastatic cancer cell adhesion, protrusion, polarity and motility than the presence of the Fn on the flat multilayer surfaces. Also, the detachement signaling mediated by ephrinB3 was found to be more effective when the ephrinB3 were modified to the nanofeatured surfaces than flat surfaces. The results in this study would give insights on the basic understanding of tumor metastasis regulated by extracellular environmental signals. In Chapter 5, we have developed novel multilayered co-culture platforms with nanoporous cellulose acetate (CA) membranes for efficient in vitro analysis of cell-cell communications. The CA films designed in this chapter have high number density of well-defined nanopores and unique natures of transparency and transferability in cell culture environments. The transparent, transferable and nanoporous (TTN) CA membrane platforms allow for imaging and analyzing cells on each layer as well as mediating the paracrine communications between co-cultivated cells. The communications between human breast metastatic cancer cell (MDA-MB-231) and three different types of stromal cells [fibroblast (NIH-3T3), myoblast (C2C12), and human mesenchymal stem cell (hMSC)] via the TTN membrane were systematically investigated by cytokine and cell migration assays based on the high flexibility in stacking and destacking of the TTN platforms. The TTN membranes would address the issues from conventional membrane-separated cell co-culture platforms that lack the routes for cell-cell communications and direct cell-cell contact assay that does not offer the flexibility in studying cell-cell communications. We strongly believe that the research work in this thesis as to the engineering of the nanostructures and physicochemical properties in polymer thin films could eventually contribute to explore new perspectives on functional multilayer films and could also open up new possibilities to design flexible and multifunctional structures for numerous biological applications such as controlled release platforms and in vitro disease cell assay platforms.